Two-, three-, and four-carbon diols, as exemplified by ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 2,3-butanediol, 1,3-butanediol, and 1,4-butanediol, find use in a myriad of industrially important polymers and formulations. Many processes have been disclosed for the production of these diols. For example, ethylene glycol can be produced by the hydrogenation of glycolic acid, glycolic acid esters, methyl glycolate, oligomers of glycolic acid, oligomers of glycolic acid ester, or mixtures thereof with a ruthenium compound such as, for example, a ruthenium-organophophorus coordination compound, that is soluble or partly soluble in the reaction mixture. Such a glycolic acid-based route has been described in the art as disclosed in U.S. Pat. No. 7,615,671. The hydrogenation of glycolic acid species may also produce by-product higher diols, primarily 1,2-propanediol and 1,2-butanediol, in small quantities, e.g., typically less than 1 weight percent of each compared to ethylene glycol.
Carbohydrate feedstocks also may be used to produce mixed diol streams with varying amounts and particular species of two-carbon diols, three-carbon diols, four-carbon-diols, and higher diols.
It is desirable to separate the mixed diol stream into its component diols in a most economic manner. Distillation, a common method of separation, can prove difficult when components have close boiling points. The normal boiling points of ethylene glycol, 1,2-propanediol, and 1,2-butanediol are 197.1° C., 187.7° C., and 196.5° C. respectively, and ethylene glycol and 1,2-butanediol are known to form a minimum boiling azeotrope, precluding complete separation by single-feed ordinary fractional distillation. Separation of such mixed diol streams into pure fractions by ordinary single-feed fractional distillation is extremely difficult, if not impossible due to numerous azeotropes and close boiling points.
There is a need for a process whereby a diol mixture can be separated into pure fractions in an efficient and cost-effective manner.